T3 L3: Formation of urine Flashcards
What is the normal GFR?
125 mL/min
What is ultrafiltration?
Filtration on a molecular scale
What are the 3 layers that filtrate has to pass through during glomerular filtration?
- Pores in the glomerular capillary endothelium
- Basement membrane of Bowmans capsule
- Epithelial cells of Bowmans capsule (the podocytes)
What is the function of Mesangial cells and where are they found?
They contract and relax to allow molecules to go through the basement membrane of the Bowmans capsule
What is the function of Podocytes and where are they found?
They cover glomerular capillaries and have pedicels that interconnect but leave small gaps so that molecules can get through
How does diabetes damage podocytes?
Podocytes are very sensitive and high [glucose] is toxic to them so with diabetes, they die, break off and end up in urine which leaves gaps in the glomerular membrane
What is oncotic pressure?
Pressure induced by proteins
What are the 3 pressures that affect glomerular filtration?
- Pressure within the glomerular capillary
- Plasma protein pressure
- Pressure within the Bowmans capsule
How does the GFR remain constant even when systemic blood pressure changes?
Because of autoregulation of the renal blood flow. This process is not regulated by hormones or neurones
What are the 2 hypotheses behind autoregulation of renal blood flow?
- Myogenic. In response to the renal arterioles stretching
- Metabolic. Renal metabolites modulate afferent and efferent arteriole contraction and dilation
But it’s believed that the process needs both
What is the macula densa?
The part that detects the changes in Na+ and has an effect on blood pressure
How can changes in GFR alter systemic blood pressure?
- A drop in filtration pressure causes a drop in GFR
- Lower GFR means less Na+ enters the proximal tubule
- Tubular Na+ changes are detected by the macula densa
- Juxtaglomerular cells will release renin in response to this
- Renin causes formation of angiotensin II
- Angiotensin II is a vasoconstrictor that causes BP to increase
- Increased BP causes filtration pressure to increase and the GFR goes back to normal
What do -pril drugs do?
They’re ACE inhibitors
What do -sartan drugs do?
They’re angiotensin receptor blockers (ARB’s)
What do -kiren drugs do?
They inhibit renin
What does the Na+/K+ ATPase do?
It pumps Na+ against the chemical gradient into the peritubular capillary (3 Na+ leave, 2 K+ enter)
Which ion follows Na+ to always balance out the charges?
Cl- by facilitated diffusion, and water
Why do proximal tubule (PT) cells have a low intracellular [Na+]?
Because the Na+/K+ ATPase pumps all the Na+ out in return for K+
What % of water is reabsorbed in the proximal tubule (PT)?
60-70%, active transport of Na+ out of PT cells is the driving force
What structures allow for transcellular water absorption and where are they found?
Aquaporin channels found on the apical and basolateral surfaces
Is there any active water reabsorption in the nephron?
None. It all occurs by osmosis and it follow Na+
How many types of aquaporins (AQP) are found in the kidneys?
4
Where are AQP1 found?
On the proximal tubule and other parts where water is reabsorbed. They are found on the apical membranes
Where are AQP2 found?
On the collecting ducts on apical surfaces
Expression of which AQP is controlled by ADH?
AQP2
Where are AQP3/4 found?
On the basolateral surface of tubular cells involved in water reabsorption
Where is glucose reabsorbed in the nephron?
The proximal tubule
How is glucose transported in the nephron?
It’s co-transported into the proximal tubule cell with Na+
What is the difference between SGLT1 and SGLT2?
SGLT2 has a lower affinity but higher capacity and transports 90% of the glucose. SGLT1 has a higher affinity but low capacity and only transports 10% of glucose
What is a SGLT?
A sodium-glucose transporter
What happens to SGLT when there is too much glucose?
They can’t handle it so some glucose will be excreted
What do -flozin drugs do?
They are taken by type 2 diabetics to block SGLT2 so more glucose can be excreted and therefore blood glucose will be reduced. It can be used as a weight loss drug is hypoglycaemia is dangerous
What % of K+ is reabsorbed by the PT?
70%
What % of urea is reabsorbed by the PT?
40-50%
What process reabsorbs proteins into to PT?
Receptor-mediated endocytosis
Describe the process of protein reabsorption in the PT
Proteins are reabsorbed by pinocytosis, vesicles are transported into the cell, degraded by lysosomes and amino acids are then returned into the blood
What is pinocytosis?
Ingestion into a cell by budding of vesicles from the membrane
